The invention relates to a process for receiving spread spectrum signals in which a receiving signal is received from a sending signal, which is encoded with a spread code, and is filtered. After this, in a first process part in a first RAKE finger, the filtered receiving signal, in an on-time correlation, is multiplied by a conjugated complex spread code, which is delayed by a base time delay in relation to the spread code of the sending signal, and the result is summed in a first adder. The result of the first summation is output as an information signal and the adder is reset. The filtered receiving signal is further processed in two alternative processes. In the first process, in a late correlation, the filtered receiving signal is multiplied by the conjugated complex spread code of the sending signal, which is delayed in relation to the spread code (c (t)) of the sending signal by a first base time delay and a positive additional time delay that increases the first base time delay, and is summed in a second adder. In an early correlation, the filtered receiving signal is multiplied by the conjugated complex spread code, which is delayed in relation to the spread code (c (t)) of the sending signal by the first base time delay and a negative additional time delay that decreases the first base time delay, and is then summed in a third adder. Then a correlation difference signal is determined as the difference between the results of the early and late correlations and the second and third adders are reset.
In the second process, the filtered receiving signal is multiplied by the difference between the conjugated complex spread code, which is delayed in relation to the spread code of the sending signal by the first base time delay and the positive additional time delay that increases the first base time delay, and the conjugated complex spread code, which is delayed in relation to the spread code of the sending signal by the first base time delay and the negative additional time delay that decreases the first base time delay, and the multiplied signal is summed in a fourth adder. This produces the correlation difference signal. After it is processed further, the fourth adder is reset. Then the real component of the difference signal, which is determined in the one or the other process, and the information signal is established as an error signal and used to control the magnitude of the base time delay.
Parallel to the above processing in a second RAKE finger, the same process steps are executed with a second base time delay which has a difference in relation to the first base time delay, and the information signals of the parallel executed processed parts are combined.
The invention also relates to a method which has the same sequence; but wherein, the receiving signal is not filtered but is instead multiplied by a filtered conjugated complex spread code, and the product signals are integrated, which achieves the correlation.
The process according to the invention uses RAKE receivers of the kind described in R. Price, P. E. Green, Jr., [In English:] xe2x80x9cA Communication Technique for Multipath Channels,xe2x80x9d Proc. IRE, Vol. 46, March 1958, pp. 555-570. The RAKE receiver is a receiver apparatus which is outstandingly suited for receiving spread spectrum signals and is used for this. The conventional RAKE receiver is comprised of a number of correlators which despread the spread spectrum signal with a different time offset and recover the narrowband signal.
Spread spectrum techniques of the type mentioned the beginning, as are described in R. L. Pickholtz, D. L. Schilling, L. B. Milstein, [In English:] xe2x80x9cTheory of Spread Spectrum Communicationsxe2x80x94a Tutorial,xe2x80x9d IEEE Transactions on Communications, Vol. COM-30, May 1982, pp. 855-884 and in M. J. Goiser Alois, Handbuch der Spread-Spectrumxe2x80x94Technik [Handbook of Spread Spectrum Engineering], Springer, 1998, have been used in the past exclusively for military applications for encoding and masking signals and for increasing jamming resistance. In this connection, a narrowband signal to be transmitted is multiplied through multiplication with a broadband pseudo-random spread sequence. The elements of the random sequence are referred to as chips. The resulting signal is likewise in broadband format. In other words, the signal to be sent, i.e. the sending signal, is encoded with a spread code.
In the receiver, this sending signal is received and then processed further as a receiving signal. After a time which is defined by the sending signal, the same broadband pseudo-random spread sequence is used in the receiver as is used for encoding the transmitted signal. This is possible because of the character of the pseudo-randomness, as a result of which the same pseudo-random sequence can be produced using the same technical means and assumptions. The sender and the receiver only need to know the means and assumptions for producing the pseudo-random sequence.
The original narrowband signal is then recovered in the receiver through multiplication by the conjugated complex spread.
In cellular mobile telephony, where a limited bandwidth must be made available to numerous subscribers, this process is likewise attractive. In this instance, different subscribers are simply associated with different pseudo-random spread sequences. For the receiver which uses the spread code of the desired subscriber to be detected, the signals of all the other subscribers behave like noise.
The information to be transmitted can be recovered in the receiver as long as the overall power of the interfering signals is compatible.
For a few years, spread spectrum has been used successfully in the American mobile telephone standard xe2x80x9cIS-95.xe2x80x9d Direct sequence spread spectrum has been proposed as the basic process for the mobile telephone standard of the third generation xe2x80x9cIMT-2000xe2x80x9d and it is probable that the mobile telephone standard of the third generation will be based on this process because it permits a simple and flexible allocation of the spectrum to different subscribers with different bandwidth requirements.
In mobile telephony, the transmitted signal of a base station usually does not travel directly to the receiver but instead arrives by a circuitous route through multiple reflections. The received signal is distinguished by an overlapping of these multiple reflections, which differ only in value, phase, and the transit time delay corresponding to the propagation path. Each signal component that has reached the receiver via reflections is in turn comprised of a series of separate signals with slight transit time differences so that the signal component that has reached the receiver via a particular path is subject to rapid fading.
Because of the favorable correlation properties of the spread spectrum signals, specific individual paths (signal components) of a multipath signal can be detected with a RAKE receiver through correlation with correspondingly delayed pseudo-random spread sequences. A combination of the correlation results permits a more reliable reconstruction of the information of the sending signal than when only a single correlation result is used.
Conventional processes use a time error estimator for each correlator of a RAKE receiver, which estimator, through a first correlation with an additional positive time offset and through a second correlation with a negative additional time offset, estimates the time delay in relation to the optimal time offset for the local random code generator, for which the actual correlator extracts the maximal signal strength from the multipath signal component. The correlator and time error estimator are often combined into one overarching unit which can also contain other estimators and which is referred to as a RAKE finger. The above-described process for time error estimation is therefore referred to as the early-late process. The estimated time delay is used by the RAKE finger itself or by an additional overarching unit for time tracking, for the so-called fine time synchronization.
Conventional, useful implementations of the early-late time error estimator function successfully with only one additional correlator for time error estimation. The receiving signal is first band limited by a receiving filter. The receiving filter is a root Nyquist filter which is adapted to the sending pulse of the broadband spread signal.
After scanning with the chip rate, only one summation of the products of the scanned values with the corresponding elements of the conjugated complex spread sequence is required for correlation and despreading.
The result of the adder is read out every N values and the adder is reset, wherein N represents the number of values allotted to a data symbol. The value thus obtained for each sum of N values is the estimated value of the information signal for the RAKE finger.
For the early-late process, in parallel to this correlation with an offset, a correlation is executed with the difference between the conjugated complex spread code that is delayed by a half-chip duration and the conjugated complex spread code that leads by half a chip duration. The real component of the product of this correlation result and the conjugated complex correlation result for the estimate value of the RAKE finger supplies the error signal that can be used for the fine time synchronization of the RAKE finger.
As a rule, the fine synchronization is achieved by virtue of the fact that the error signal is transmitted through a narrowband loop filter and the filtered signal controls the local spread code generator.
In the known process for fine time synchronization, it is disadvantageous that the early-late processes used for time error estimation are in fact optimal for channels with only one path, but are very interference-prone with multipath signals which differ by only a slight time offset (slight in comparison to the clock speed of the random sequence), i.e. when the differences of the circuitous transit times can no longer be solved due to the band limitation of the signals. On the one hand, the sluggishness of the control loops involved in the fine synchronization is designed for the changeability of the relative time offsets of multipath signals, but it is not designed for the changeability of the rapid fading caused by movement so that with faster movement, it is not possible to track the optimal time offset. On the other hand, there is the possibility that with independent regulation, due to the multipath profile, chronologically adjacent RAKE fingers have the same time offset. This is undesirable to the extent that in this instance, no additional information is obtained from the receiving signal. When there is a relative time offset of fewer than one cycle duration of the random sequence, then it can be assumed that the output data of the RAKE fingers are correlated. In known processes, therefore, one of two chronologically adjacent RAKE fingers is switched off when a previously determined minimal time difference between the RAKE fingers has not been met due to the individual fine synchronization. In certain cases, additional information would be easily obtainable from another RAKE finger with the permitted minimal time offset.
The object of the invention, therefore, is to disclose a process for receiving spread spectrum signals with which the signal to be transmitted can be generated from the receiving signal with the greatest possible signal-to-noise ratio.
According to the invention, this object is attained by virtue of the fact that at the beginning of the process, the amount of the difference between first and second base time delays in RAKE fingers is greater than a minimum difference and that when the minimum difference is reached during the course of the process, the error signals of the first process part and the second process part are combined. This combined error signal is used as a new error signal for controlling the base delay in both process parts.
The advantage of the method according to the invention is comprised in that with channels in which the transit time differences of the multipath paths approximately correspond to the chip duration, or are less than it, i.e. can no longer be resolved by the receiver, still more RAKE fingers can be used in order to obtain more information from the receiving signal about the sending signal than is possible with conventional processes. This means that the information contained in the multipath signal components can still be effectively used even if a precise determination of the paths is no longer possible due to the transit time differences. The average synchronization error turns out to be lower for the apparatus according to the invention.
The variance of the time offset for one group of RAKE fingers with unresolvable multipath signals is lower with the fine synchronization process according to the invention than is the case with a single RAKE finger.
The process finds the time offset for the group of RAKE fingers for which the maximal ratio combining (MRC), as a process for combining the correlation results of the RAKE fingers, provides the maximal signal-to-noise ratio. MRC is the process that provides the maximal signal-to-noise ratio when combining input signals with uncorrelated noise.
The process is non-coherent, i.e. information about the carrier phase is not required for fine synchronization. As a result, the process is not assigned to additional channel estimation processes whose function does not always have to be assured in multipath fading channels. The process is equally suited for coherent and non-coherent transmission processes.
The process is based on a non-coherent process for finding the optimal time offset of a correlator in the absence of multipath propagation, which correlator functions successfully with only one additional correlator (FIG. 1). Furthermore, correlation is carried out with the difference code which in the direct sequence spread spectrum is comprised on average of 50 percent zeros, which permits an implementation in which the power consumption is reduced by up to 50 percent.
A favorable embodiment of the process according to the invention provides that the minimum difference corresponds to one chip duration.
As a result, the noise of information signals is uncorrelated in both process parts.
Another embodiment of the process according to the invention provides that when an originally large difference between the first and second base time delays is reduced to the minimum difference during the course of the process, the first and second RAKE fingers are grouped. From then on, the process steps are executed jointly for the group.
In this connection, it is possible to improve the process according to the invention by virtue of the fact that a RAKE finger is removed from the group when, during the course of the process steps, the difference between the first and second base time delays would change to a value greater than the minimum difference with separate error signals.
It is also possible that from the beginning of the process progression onward, a group is initiated independent of the difference between the first and second base time delays.
As a result, data from neighboring multipath paths can be used from the start.
In this connection, it is to be expected that the grouping reduces the variance of the regulating signal markedly, which can be explained based on the improved utilization of the multipath channel by the apparatus. To this end, another embodiment of the process provides that the first RAKE finger has a second RAKE finger added to it, whose base delay differs from that of the first RAKE finger by the minimum interval, as a result of which the first and second RAKE fingers constitute a group, or a third RAKE finger with a minimum interval is added to a group of RAKE fingers, as a result of which the number of RAKE fingers in a group is increased.
It is probable that neighboring multipath paths are not always recognized by the acquisition unit and initially, only a single RAKE finger is used for the detection of these paths. Due to the changeability of the channel produced by the rapid fading, in this instance, an increased variance of the time error signal or the time offset of the RAKE finger must be expected. Through an observation (measurement) of this variance, it is possible to detect closely adjacent multipath propagation. As a result, an additional RAKE finger can be assigned to the first one, whose base delay differs from the first one by the minimum interval, as a result of which the two RAKE fingers constitute a group and the desired result is achieved.
The invention will be explained in detail below in conjunction with two exemplary embodiments.